1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile machine, and the like, that forms an image utilizing an electrophotography system.
2. Related Background Art
In FIG. 9, a structure of a so-called tandem type color image forming apparatus is schematically shown. The tandem type color image forming apparatus has a plurality of image forming portions that form toner images of different colors, respectively, where a color image is formed by sequentially superposing the individual color images on a transferring material. In the image forming portions Pa, Pb, Pc and Pd, toner images of respective colors, i.e., magenta, cyan, yellow and black, are formed in accordance with an electrophotography system.
In the respective image forming portions Pa, Pb, Pc and Pd, upon emission of a signal for starting an image forming operation, photosensitive drums 1a, 1b, 1c and 1d, which are driven to rotate at a specific process speed, are charged by charging rollers 2a, 2b, 2c and 2d, respectively. On the other hand, exposing apparatuses 6a, 6b, 6c and 6d convert input image signals that have been separated for respective colors into light signals. The photosensitive drums 1a, 1b, 1c and 1d that have been charged as above are exposed to laser beams modulated as light signals so as to be scanned, respectively, whereby electrostatic latent images are formed on the photosensitive drums 1a, 1b, 1c and 1d. 
The electrostatic latent image formed on the photosensitive drum 1a is first visualized (developed) as a toner image by applying yellow toner onto the electrostatic latent image using a developing apparatus 3a. 
In synchronization with this visualization (development), a transferring material P, such as a paper sheet, which is electrostatically absorbed to a surface of a transferring material conveying belt 9, and which is moved by a driving roller 10a, is conveyed to a transferring portion located between the photosensitive drum 1a and a transferring blade 4a of the image forming portion Pa, whereby the yellow toner image on the photosensitive drum 1a is transferred onto the transferring material P with the aid of the transferring blade 4a, to which a transferring bias is applied. The transferring material conveying belt 9 is looped around the driving roller 10a and a driven roller 10b. 
Similarly, in the image forming portion Pb, a magenta toner image formed on the photosensitive drum 1b is transferred with the aid of a transferring blade 4b, to which a transferring bias is applied, onto the transferring material P in such a way that the magenta toner image is superposed on the yellow toner image on the transferring material P. Subsequently, cyan and black toner images formed on the photosensitive drums 1c and 1d are sequentially transferred with the aid of transferring blades 4c and 4d, to which a transferring bias is applied, onto the transferring material P in such a way that the cyan and black toner images are superposed on the yellow and magenta toner images that have been superposed on the transferring material P. Thus, a full color toner image is formed on the transferring material P.
The transferring material P, on which the full color toner image has been transferred, is conveyed to a fixing apparatus 13. The fixing apparatus 13 applies heat and pressure to the transferring material at a fixing nip between a fixing roller 13a and a pressurizing roller 13b, so that the full color toner image is fixed on the transferring material P. After the fixing operation, the transferring material P is discharged from the apparatus. In the above transferring process, residual toner remaining on the photosensitive drums 1a, 1b, 1c and 1d is removed and collected by cleaning apparatuses 5a, 5b, 5c and 5d. 
Recently, a contact transferring member, such as a transferring blade or a transferring roller, has become widely used as transferring means of a contact transfer scheme, as is the case in the above-described example of the image forming apparatus. The contact transferring member is adopted in view of its advantageous stability in transferring images. A contact transfer system that uses a contact transferring member such as a transferring blade or a transferring roller has advantages in that a transferring voltage is reduced, as compared to a conventional corona transfer system, and the amount of ozone generated during the process is small.
As means for applying transferring bias to such a contact transferring member, a constant current system that applies a constant current is sometimes used, in order to cope with variations, such as resistance variations due to environmental variations, in the electrical resistance of the contact transferring member.
The constant current application system as described above suffers from the following problems.
In a portion of a photosensitive drum extending along the longitudinal direction of the drum that is located at a transferring portion at which a contact transferring member (i.e., transferring blade or transferring roller, etc.) abuts the photosensitive drum, a relatively large transferring current flows in a portion (or area) in which a toner image is absent; it is more difficult for current to flow in a portion in which a toner image is present due to the toner""s resistance. Therefore, when an image in which an image ratio (or percentage of image) along the longitudinal direction of the photosensitive drum is high is to be formed, a high quality image that does not involve transferring errors would be obtained with a small transferring current; on the otherhand, when an image in which an image ratio along the longitudinal direction of the photosensitive drum is low is to be formed, a relatively large transferring current is required in order to obtain a high quality image.
In such an image forming apparatus, when the transferring current is set to a value optimum for images in which the image ratio along the longitudinal direction of the photosensitive drum is high, transferring errors would occur upon forming images in which the image ratio along the longitudinal direction of the photosensitive drum is low, due to a shortage of the transferring current. On the other hand, when the transferring current is set to a value optimum for images in which the image ratio along the longitudinal direction of the photosensitive drum is low, transferring errors such as inversion in the polarity of toner would occur upon forming images in which the image ratio along the longitudinal direction of the photosensitive drum is high due to an excessive current flow in the toner image.
In view of the above situations, a method has been conceived that controls the transferring current in accordance with the image ratio so as to avoid variations in transferring performance due to variations in the image ratio, as described in, for example Japanese Patent Application Laid-Open No. 08-083006.
However, in an image forming apparatus like the apparatus shown in FIG. 9, which forms a color image by superposing toner images of different colors, a preceding color toner image(s) already is present on the transferring material when a subsequent color image is transferred onto the transferring material, so transferring errors cannot be fully avoided by taking into account only the image ratio of a monochromatic (or single-color) toner images, like in the above-mentioned Japanese Patent Application Laid-Open No. 08-083006, and transferring errors can still occur.
In the following, the way in which such transferring errors occurs will be described, referring to FIGS. 10A and 10B.
As shown in FIG. 10A, in the image forming portion Pa a first color toner image (i.e., yellow toner image) denoted by t is formed on the photosensitive drum 1a at about the center with respect to the longitudinal direction of the drum 1a. Then, as shown in FIG. 10B, in the image forming portion Pb, a second color toner image (i.e., magenta toner image) t1 formed on the photosensitive drum 1b is transferred so as to be superposed on the first color toner image (or yellow toner image).
In doing so, according to the method disclosed in Japanese Patent Application Laid-Open No. 08-083006, a common or the same setting of the transferring current is used in both the transferring processes shown in FIGS. 10A and 10B. However, in the case shown in FIG. 10B, the value of a current that actually flows through the second color toner image t1 differs from that value in the case of FIG. 10A due to the presence of the first color toner image t in FIG. 10B.
As per the above, in the transferring current setting operation described in Japanese Patent Application Laid-Open No. 08-083006, the transferring current is appropriate only for the first color toner image t in FIG. 10A; it is too large for the second color toner image t1 in FIG. 10B, in which the first color toner image t is present as well as the second color toner image t1. This excessive current would cause transferring errors, such as inversion of toner polarity, etc.
On the other hand, an optimum value for the transferring current varies depending on the type of the transferring material, such as paper, or on variations in the electrical charge amount of the toner due to environmental variations of the image forming apparatus. Therefore, such methods as described in Japanese Patent Application Laid-Open No. 08-083006 that take into account only the image ratio of a monochromatic toner image suffer from a problem in that transferring errors due to variations in the electrical charge amount of the toner can occur.
Japanese Patent Application Laid-Open No. 10-78691 discloses another prior art system. When red and black toner images that have been multiple-transferred on a photosensitive drum are to be transferred onto a transferring material at the same time by a corona transferring charger, optimum transferring points for respective images that have different electrical charges of the toner are different from each other. This prior art focuses on avoiding transferring errors due to the difference in the optimum transferring points upon transferring the toner images at the same time. Japanese Patent Application Laid-Open No. 10-78691 teaches a method of determining an optimum transferring current based on the respective image ratios of red and black, and the ratio (or proportion) of the overlapping of the images. However, the prior art disclosed in Japanese Patent Application Laid-Open No. 10-78691 is not directed to a contact transfer scheme; it has nothing to do with the transferring error problem related to the flow of transferring current along the longitudinal direction of the photosensitive drum, to which the present invention is directed. Therefore, there is no description related to image ratios along the longitudinal direction of the drum in Japanese Patent Application Laid-Open No. 10-78691, and the prior art disclosed therein cannot effectively overcome the problem focused on by the present invention.
In view of the above situation, an object of the present invention is to provide an image forming apparatus that can apply an optimum transferring current even upon transferring different toner images in a superposed manner.
For the above purpose, according to the present invention there is provided as a preferred aspect thereof, an image forming apparatus comprising:
an image bearing member;
image forming means for forming an image on the image bearing member;
a contact transferring member for transferring the image formed on the image bearing member onto a transferring medium;
transferring current applying means for applying a current to the contact transferring member; and
control means for controlling an amount of the current applied by the transferring current applying means;
wherein images formed on the image bearing member are transferred in a superposed manner onto the transferring medium that is being conveyed, and
wherein the control means determines an amount of current to be applied to the contact transferring member based on:
an image ratio in a direction perpendicular to a direction in which the transferring medium is conveyed, of an image that has already been transferred on the transferring medium,
an image ratio in a direction perpendicular to a direction in which the transferring medium is conveyed, of an image on the image bearing member, and
an image ratio in a direction perpendicular to a direction in which the transferring medium is conveyed, of a portion in which the images overlap.
Other object, features and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.